This invention relates to the preparation of silicone foams, and more particularly to a method for their preparation which employs microwave radiation.
As used herein, the term "silicone foam" designates a polyorganosiloxane composition in the form of a foam. Silicone foams are well known in the art and their preparation is described in a number of patents. A method of high current interest involves contact in a one-part mixture between a polyorganosiloxane having vinyl groups attached to silicon, a polysiloxane having hydrogen atoms attached to silicon and a hydroxy compound. Upon contact with a platinum group catalyst, two reactions occur: hydrosilylation of the vinyl groups by interaction with the silicon-hydrogen moieties, and generation of hydrogen by interaction of the hydroxy compound with the silicon-hydrogen moieties. The hydrogen thus generated acts as a blowing agent to produce the foam.
Numerous types of hydroxy compounds have been employed in this method of preparing foam. Water and hydroxyaliphatic compounds, including alkanols, alkanediols and the like, are often preferred. However, it is usually important for water to be present as at least part of the blowing agent.
One problem encountered in the production of silicone foams by the above-described reaction is the short working life of the mixture after all reactants have been brought into contact. Even at room temperature, hydrosilylation and hydrogen generation occur almost immediately. Thus, it is generally not possible to prepare a one-part mixture which has a useful shelf life and which can later be converted into a silicone foam.
The use of microwave radiation for silicone foam preparation has been suggested. Reference is made, for example, to U.S. Pat. No. 4,026,844, in which an acetylenic alcohol is used as a catalyst inhibitor to increase the shelf life of the one-part foam-producing composition, so that reaction will take place only upon exposure to microwave energy. Other catalyst inhibitors, such as triarylphosphines and dialkyl maleates, are also known. However, they effectively inhibit only the hydrosilation step and not the foam-producing step. Thus, foam production can take place spontaneously before microwave activation, resulting in the formation of a non-crosslinked foamed product which does not have the desired properties.
It remains of interest, therefore, to develop silicone foam-producing compositions and methods which employ microwave energy for foam production and Which permit delay of both hydrosilation and foaming until those operations can be simultaneously initiated by microwave activation. These effects are achieved by the present invention.